[C#] Less strict readonly keyword matching

This commit scopes `readonly` modifier more lazily to improve
interoperability with `required` keyword.

C#/C#.sublime-syntax

@@ -334,7 +334,7 @@ contexts:
     - include: main
 
   class_declaration:
-    - match: \b(static|unsafe|abstract|partial|required|sealed)\b
+    - match: \b(static|unsafe|abstract|partial|readonly|required|sealed)\b
       scope: storage.modifier.cs
     - match: '{{visibility}}'
       scope: storage.modifier.access.cs
@@ -343,22 +343,19 @@ contexts:
         1: keyword.declaration.class.cs
         2: entity.name.class.cs
       push: [class_signature, data_type_signature]
-    - match: (?:\b(readonly)\s+)?\b(record)\s+(?:(struct)\s+)?({{name}})
+    - match: \b(record)\s+(?:(struct)\s+)?({{name}})
       captures:
-        1: storage.modifier.cs
-        2: keyword.declaration.class.record.cs
-        3: keyword.declaration.struct.record.cs
-        4: entity.name.class.cs
+        1: keyword.declaration.class.record.cs
+        2: keyword.declaration.struct.record.cs
+        3: entity.name.class.cs
       push: [record_signature, data_type_constraint, record_parameters, data_type_signature]
-    - match: (?:\b(readonly)\s+)?(?:\b(partial)\s+)?(?:\b(ref)\s+)?\b(struct)\s+({{name}})
+    - match: (?:\b(ref)\s+)?\b(struct)\s+({{name}})
       captures:
         1: storage.modifier.cs
-        2: storage.modifier.cs
-        3: storage.modifier.cs
-        4: keyword.declaration.struct.cs
-        5: entity.name.struct.cs
+        2: keyword.declaration.struct.cs
+        3: entity.name.struct.cs
       push: [struct_signature, data_type_signature]
-    - match: '\b(enum)\s+({{name}})\s*(?:(:)\s*(byte|sbyte|short|ushort|int|uint|long|ulong))?'
+    - match: \b(enum)\s+({{name}})\s*(?:(:)\s*(byte|sbyte|short|ushort|int|uint|long|ulong))?
       scope: meta.enum.cs
       captures:
         1: keyword.declaration.enum.cs

C#/tests/syntax_test_C#10.cs

@@ -34,7 +34,7 @@ public record struct Person(string Name);
 
 public readonly record struct Person(string Name);
 ///^^^ storage.modifier.access
-///    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.class.record - meta.class.record meta.class.record
+///             ^^^^^^^^^^^^^^^^^^^^ meta.class.record - meta.class.record meta.class.record
 ///                                 ^^^^^^^^^^^^^ meta.class.record.parameters - meta.class.record meta.class.record
 ///    ^^^^^^^^ storage.modifier
 ///             ^^^^^^ keyword.declaration.class.record

C#/tests/syntax_test_C#11.cs

@@ -146,10 +146,12 @@ public class C2 {
 ///                 ^^^^^^ storage.type
 }
 
-public struct S2 {
+public readonly required struct S2 {
 /// <- storage.modifier.access
-///    ^^^^^^ keyword.declaration.struct
-///           ^^ entity.name.struct
+///    ^^^^^^^^ storage.modifier
+///             ^^^^^^^^ storage.modifier
+///                      ^^^^^^ keyword.declaration.struct
+///                             ^^ entity.name.struct
 
     public required string FirstName { get; init; }
 /// ^^^^^^ storage.modifier.access

C#/tests/syntax_test_C#7.cs

@@ -727,7 +727,7 @@ void Foo (in string s, in int x, in Point point)
 bytes[2] = 44; // throws IndexOutOfRangeException
 
 public readonly ref struct Span<T>
-///    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.struct
+///             ^^^^^^^^^^^^^^^^^^^ meta.struct
 ///  ^ storage.modifier.access
 ///    ^^^^^^^^ storage.modifier
 ///             ^^^ storage.modifier